Silicon carbide (SiC) is a high-hardness semiconductor material with a greater bandgap than silicon (Si), and has been used extensively in various types of semiconductor devices including power devices, hostile-environment devices, high temperature operating devices, and radio frequency devices. Among other things, the application of SiC to power devices such as switching devices and rectifiers has attracted a lot of attention. This is because a power device that uses SiC can significantly reduce the power loss compared to a Si power device. In addition, by utilizing such properties, SiC power devices can form a smaller semiconductor device than Si power devices.
A metal-insulator-semiconductor field effect transistor (MISFET) is a typical semiconductor element among various power devices that use SiC. In this description, an MISFET of SiC will be sometimes simply referred to herein as an “SiC-FET”. And a metal-oxide-semiconductor field effect transistor (MISFET) is one of those MISFETs.
There have been reports that if a forward current is supplied to the pn junction of SiC, stacking faults will grow due to basal plane dislocations, which is a problem unique to SiC. Such a problem will arise when an SiC-FET is used as a switching device for power converters for driving and controlling a load such as a motor. If an SiC-FET is used as a switching device for power converters, then reverse current needs to flow when the SiC-FET is in OFF state. A pn junction inside an SiC-FET is sometimes used as a path for such reverse current. Since a pn junction is present within the semiconductor element and operates as a diode, such pn junction is called a “body diode”. If the pn junction diode (body diode) of an SiC-FET is used as an inverse diode (freewheeling diode), then the diode current will flow in the forward direction of the pn junction diode. It is believed that if such current flows through the pn junction of SiC, then such bipolar operation will degrade the crystallinity of the SiC device, i.e., stacking faults grows at the pn junction (see, for example, Patent Document No. 1).
Degradation of crystallinity could impact the ON voltage of the body diode, raising it from its initial value. Also, if a body diode is used as an inverse diode, a reverse recovery current will flow due to the bipolar operation of the pn junction diode when the diode in ON state switches to OFF state. And that reverse recovery current causes not only recovery loss but also a decrease in switching rate as well.
Thus, in order to overcome such a problem involved by using a body diode as an inverse diode, it was proposed (in Patent Document No. 2, for example) that a reverse current is made to flow through an inverse diode element as an electronic part by connecting the inverse diode element and an SiC-FET in anti-parallel with each other.